When NYSE breadth misreads operating-stock participation

Exchange listings are not a single participation sample

Market breadth is often read as if the exchange listing tape were one participation fact. About 48% of NYSE-listed issues were funds or preferreds rather than operating companies, so official advance-decline totals could track interest-sensitive listings more than equity participation.

A reconstructed cumulative advance-decline line was built from an optionable-stock universe of operating companies that trade options, with only a few exchange-traded funds included.

Where the two advance-decline lines diverged

Over a window a little more than a year, the official NYSE cumulative advance-decline line and the optionable-stock line diverged from March through September 2001. The split was especially clear in July and August, when the NYSE line made new highs while the optionable line made new lows.

A year of policy rate cuts, plus decimal ticks of a penny or two on bond funds, both inflated the count of advancing non-operating issues.

Moving-average Arms windows on two universes

The Arms index combines the issue-count ratio with the inverse volume ratio. High readings mark heavy-down sessions and low readings mark heavy-up sessions.

One Arms-index convention treated a 10-day moving average above 1.20 as oversold and below 0.80 as overbought, with a 55-day average used for longer context. In summer 2001 the 10-day Arms average reached 1.5 and later 1.6, and the 55-day average stayed above 1.2 for nearly two months from July to September, while the tape was making a top rather than a bottom.

On a 21-day Arms average, a reading above 1.30 was treated as a bullish threshold. The NYSE version exceeded 1.40 in July and mid-August 2001, while a stocks-only version stayed below 1.30 and then below 1.20. The stocks-only series did not exceed 1.30 until September.

When both Arms ratios sit above one

Low-volume non-operating issues inflate the advance-to-decline issue fraction, while declining operating stocks inflate the declining-to-advancing volume fraction. Both Arms components can exceed 1.00 at once.

Issue counts, volume, and a trending bond market

Any indicator that uses raw advance and decline counts should be treated cautiously when the bond market is in a strong trend. A prolonged bond decline would bias NYSE-based readings too bearish.

Indicators that use advancing and declining volume are less exposed to this listing-mix distortion because the non-operating names contribute little volume.